In order to effect a satisfactory process in electrometallurgical furnaces and to satisfactorily utilize their electric capacity, it is a fundamental condition that the resistance factors of the furnace and of the electrode circuits are mastered. This applies both when the process heat is generated totally or partially by direct resistance heating of the charge, as with reduction and slag smelting furnaces, and when the heat is generated totally or partially by arcs, as with steel furnaces. The composition of the charge and its conductivity interplay with internal processes (e.g. varying SiO-formations) and with purely electrical conditions, resulting in changes in the positions of the electrodes and in displacement of the electrical zero point in the furnace.
Thus, the ability of the furnace to function is predominantly dependent on the ability to measure, monitor and consciously control the resistance conditions of the electrodes. Normally, endeavours are made to achieve this control by adjusting the height of the electrode tips in the furnace charge with the aid of automatic regulators.
The electrical resistances are both electrically and metallurgically dependent. Optimum furnace performance therefore presumes contemporary electrical and metallurgical stability. It is imperative that a metallurgical balance prevails, if the electrical capacity of the furnace is to be utilized to the full. The mutual relationships of the electrical circuits and the position and stability of the electric zero point are also of special importance.
Consequently, with respect to electrode control it is particularly important to be able to use control magnitudes which correctly reflect the electrical resistance conditions.
Attempts have been made previously to control the electrodes with the aid of various control magnitudes:
Constant current levels in the electrodes: This control method, which is rather often used, does not afford stability in the electrode positions when variations in voltage occur, but leads to so-called "dancing electrodes". PA1 Constant electrode power: This control method affords even less stability. PA1 Constant resistance: So-called resistance control has been considered the most reliable control method, but assumes that the electrical resistance between electrodes and the electrical zero point can be measured with sufficient accuracy. While this is simple in the case of single-phase furnaces, it is more complicated in the case of three-phase furnaces, because of the difficulty of access of the zero point.
In a number of processes involving a highly conductive metal bath (e.g. pig-iron), where the zero point is in all probability practically anchored within the bath, it has been possible passably to consider the furnace as three single-phase furnaces connected in parallel. In such cases it has been endeavoured to place a measuring zero at the bottom of the furnace, which is then considered to coincide relatively closely to the true zero point of the system. By measuring phase voltage and phase current of respective electrodes directly (with respect to the bottom zero point) there are obtained approximatively usable auxiliary values of the resistance of the crater zones, when the remaining furnace running conditions are carefully upheld.
This procedure is not practicable, however, in other highly important processes (high percentage Si-alloys, slag smelting etc.). The nature of the furnace as a three-phase furnace (as distinct to three parallel single furnaces) appears clearly in these cases. The zero point is by no means reliably anchored, but is able to wander readily in relation to the electrode tips, so-called "fluttering zero point". As with the aforementioned "dancing electrodes" this often results in highly instable electrode positions and resistance values. In those cases when a rotating furnace body is used, there is also the purely constructional problem of incorporating a measurement zero point in the furnace structure.
It can thus be said that the inability to discern and to render more precise, in a measurement technical fashion, those furnace parameters which, inter alia, are necessary in order to enable control of the electrodes to be effected in a satisfactory manner, has hitherto constituted a significant problem in the art with respect to the practical performance of the process.